The present invention relates to an information recording apparatus which utilizes light beams to write information onto information storage media.
The present application claims priority from Japanese Application No. 2002-165779, the disclosure of which is incorporated herein by reference.
A conventional information recording apparatus for writing information onto a write-once or rewritable information storage medium (hereinafter generally referred to as an “optical disc”) includes various types of servo systems APC and FT as shown in the block diagram illustrated in FIG. 7.
The servo system APC provides feedback control on the power supplied to a semiconductor laser LD provided in a pickup PU, in order to set the laser beam (hereinafter referred to as a “write laser beam”) Po emitted from the semiconductor laser LD at a predetermined target value.
The servo system FT performs focus servo and tracking servo to properly illuminate the recording surface of the optical disc with the write laser beam Po.
More specifically, the pickup PU is provided with the aforementioned semiconductor laser LD, a light-receiving device PD, a light-receiving device OEIC having a plurality of photoelectric transducers split into a plurality of planes, a half mirror HM, and a lens assembly (shown without reference symbols). The pickup PU also includes a focus actuator F for focusing the write laser beam Po onto the recording surface of the optical disc DSC and a tracking actuator T for positioning the write laser beam Po over the desired track on the recording surface.
For example, when write data Dwt such as image data or audio data and a clock signal CK for setting the timing of a write operation are supplied to a pulse generator 1, a write signal NRZI corresponding to the write data Dwt is generated in sync with the clock signal CK.
A drive circuit 2 supplies a drive current Iwt corresponding to the level of the write signal NRZI to the semiconductor laser LD, thereby allowing the semiconductor laser LD to emit the write laser beam Po corresponding to the write data Dwt.
The write laser beam Po is reflected off the half mirror HM with a predetermined reflectivity and then passes through the aforementioned optical system to impinge upon the recording surface of the optical disc DSC, thereby allowing information to be written thereon.
The light-receiving device PD receives part of the write laser beam Po (a transmitted beam) which has passed through the half mirror HM, thereby outputting a detection signal Sapc indicative of the power (light intensity) of the write laser beam Po.
The plurality of photoelectric transducer planes of the light-receiving device OEIC receive, through the half mirror HM, reflected beams caused when the write laser beam Po impinges upon the optical disc DSC, thereby outputting a detection signal Sft containing information on focus and tracking errors.
The servo system APC allows an I-V (current to voltage) converter 3 to convert the detection signal Sapc into a detection signal Vapc, and then allows a sample and hold circuit 4 to sample and hold the detection signal Vapc, thereby supplying the resulting detection signal Vsh to a feedback control circuit 5.
The feedback control circuit 5 then determines the difference ΔV between a predetermined target value and the detection signal Vsh to make fine adjustments to the level of the drive current Iwt output from the drive circuit 2 so that the difference ΔV takes on a predetermined value (e.g., zero), thereby allowing the semiconductor laser LD to emit the write laser beam Po at an appropriate power.
The servo system FT has a tracking error generation circuit 6, a focus error generation circuit 7, and sample and hold circuits 8, 9, with the generation circuits 6 and 7 formed as circuitry having operational functions and a plurality of subtractors and adders connected in multiple stages.
The tracking error generation circuit 6 allows the plurality of subtractors and adders to perform operational processing on a plurality of detection signals Sft output from the plurality of photoelectric transducer planes provided in the light-receiving device OEIC, thereby generating a tracking error signal Ste. The sample and hold circuit 8 then samples and holds the tracking error signal Ste at a predetermined timing, thereby outputting a tracking servo signal TE for providing servo control to the tracking actuator T.
The focus error generation circuit 7 allows a plurality of differential amplifiers and adders to perform operational processing on a plurality of detection signals Sft, thereby generating a focus error signal Sfe. The sample and hold circuit 9 then samples and holds the focus error signal Sfe at a predetermined timing, thereby outputting a focus servo signal FE for providing servo control to the focus actuator F.
In this step, the pulse generator 1 outputs a sample and hold signal SHP in sync with the write signal NRZI, and the sample and hold circuits 4, 8, and 9 perform the aforementioned sample and hold operation in sync with the sample and hold signal SHP, so that the detection signal Vsh, the tracking servo signal TE, and the focus servo signal FE are generated at a predetermined timing.
For example, recent developments in information multimedia have raised an increasing demand for writing larger amounts of data at higher speeds. To meet this demand, it has become extremely critical to provide further improved performance to power control of the semiconductor laser and to servo systems such as focus servo or tracking servo.
However, the prior art servo systems APC and FT shown in FIG. 7 had a problem that increasing the frequency fck of the clock signal CK to write information at higher speeds (i.e., shortening the period 1/fck) would degrade the accuracy in servo control.
That is, differences were found between the delay time required for the I-V converter 3 to perform current-to-voltage conversions of the detection signal Sapc into the detection signal Vapc, the delay time required for the tracking error generation circuit 6 to generate the tracking error signal Ste from the detection signal Sft, and the delay time required for the focus error generation circuit 7 to generate the focus error signal Sfe from the detection signal Sft.
This made it difficult to properly extract information about power variations, tracking error, and focus error even when the sample and hold circuits 4, 8, and 9 sample and hold each of the detection signal Vapc, the tracking error signal Ste, and the focus error signal Sfe in sync with the sample and hold signal SHP generated by the pulse generator 1. For this reason, there was a problem of a significant error in accuracy of each of the aforementioned power control, the tracking servo, and the focus servo, which are performed using the detection signal Vsh, the tracking servo signal TE, and the focus servo signal FE, output from the sample and hold circuits 4, 8, and 9, respectively.
FIGS. 8A and 8B are timing charts each illustrating exemplary waveforms of the write signal NRZI, a variation in the power of the write laser beam Po, the sample and hold signal SHP, the detection signal Vapc, and the tracking error signal Ste, FIG. 8A showing a case where information is written not at a high speed and FIG. 8B showing a case where information is written at a high speed.
As can be seen from FIGS. 8A and 8B, in the case where information is written not at a high speed (as shown in FIG. 8A), the detection signal Vapc and the tracking error signal Ste have already been generated to be at a stable level by the point in time ti at which the sample and hold circuits 4 and 8 sample and hold the same in sync with the sample and hold signal SHP. As a result, sampling and holding the signals Vapc and Ste at the time point ti makes it possible to generate the detection signal Vsh having information about power variations and tracking error, and to generate the tracking servo signal TE.
In contrast to this, in the case where information is written at a high speed (as shown in FIG. 8B), the tracking error signal Ste may not have been generated by the point in time ti at which the sample and hold circuits 4 and 8 perform the sample and hold operation in sync with the sample and hold signal SHP, even though the detection signal Vapc may have already been generated by the point in time ti.
Thus, for example, the sample and hold operation at the point in time ti may allow the detection signal Vsh having information about variations in power to have been generated, whereas allowing the tracking servo signal TE having information about tracking error not to be yet generated. This led to a significant error in control accuracy for each of the servo systems APC and FT, resulting in the failure of the entire information recording apparatus to properly write information.